Thermostats
Automatic
control of the temperature of the engine is necessary for efficient engine performance and
economical operation. If the engine is allowed to operate at a low temperature, sludge
buildup and excessive fuel consumption will occur. On the other hand, overheating the
engine or operating it above normal temperature will result in burnt valves and faulty
lubrication. The latter causes early engine failure.
The thermostat
senses engine temperature and controls coolant flow through the radiator. It allows
coolant to circulate freely only withinthe blockuntil the desired temperature is reached.
This action shortens the warm-up period. The thermostat normally fits under the thermostat
housing between the engine and the end of the upper radiator hose. The pellet-type
thermostat that is used in modern pressurized cooling systems incorporates the piston and
spring principle (fig. 6-11).
The thermostat
consists of a valve that is operated by a piston or a steel pin that fits into a small
case, containing a copper impregnated wax pellet. A spring holds the piston and valve in a
normally closed position. When the thermostat is heated, the pellet expands and pushes the
valve open. As the pellet and thermostat cools, spring tension overcomes pellet expansion
and the valve closes.
Thermostats
are designed to open at specific temperatures. This is known as thermostat rating. Normal
ratings are between 180°F and 195°F for automotive applications and between 170°F and
203°F for heavy-duty applications. Thermostats will begin to open at their rated
temperature and are fully open about 20°F higher, For example, a thermostat with a rating
of 195°F starts to open at that temperature and is fully open at about 215°F.
Most engines
have a small coolant bypass passage that permits some coolant to circulate within the
cylinder block and head when the engine is cold and the thermostat is closed. This
provides equal warming of the cylinders and prevents hot spots. When the engine warms up,
the bypass must close or become restricted. Otherwise, the coolant would continue to
circulate within the engine and too little would return to the radiator for cooling.
The bypass
passage may be an internal passage or an external bypass hose. The bypass hose connects
the cylinder block or head to the water pump. There are two internal bypass systems that
can be used on an engine.
One internal
bypass system uses a small, spring-loaded valve located in the back of the water pump. The
valve is forced open by coolant pressure from the pump when the thermostat is closed. As
the thermostat opens, the coolant pressure drops within the engine and the bypass valve
closes.
Another bypass
system has a blocking-bypass thermostat (fig. 6-12).
This thermostat operates as previously described, but it also has a secondary, or bypass,
valve. When the thermostat valve is closed, the circulation to the radiator is shut off.
However, when the bypass valve is open, coolant is allowed to circulate through the
bypass. As the thermostat valve opens, coolant flows into the radiator and the bypass
valve closes.
Some
stationary engines and large trucks are equipped with shutters that supplement the action
of the thermostat in providing a faster warm-up and in maintaining proper operating
temperatures. When the engine coolant is below a predetermined temperature, the shutters,
located in front of the radiator, remain closed and restrict the flow of air through the
radiator. Then as the coolant reaches proper temperature, the shutters start to open. Two
methods are used to control the shutter opening. A stationary engine uses a shutterstat
(long thermostatic valve) connected to the engine cooling system with hoses or pipes that
allow the coolant to circulate through the valve. The temperature of the coolant, when it
reaches a predetermined temperature, causes the valve to expand extending a rod which
through linkage forces the shutters open. Trucks, equipped with an air brake, use a
smaller thermostatic valve that actuates an air valve. This air valve allows pressure from
the air tank to enter the air cylinder attached to the shutter operating mechanism,
forcing the shutters open. |
